A concrete shed base should be 100mm thick on 100mm of compacted sub-base, sized to match the shed footprint exactly rather than overhanging it. For a typical 8 × 6 ft shed that’s about 0.43m³ of concrete — well within bag or volumetric territory.
Sizing the base
Match the base to the shed’s floor dimensions, not its roof. Sheds are usually quoted by nominal size, and the actual floor is often 50 to 75mm smaller each way, so measure the base of the shed itself or check the manufacturer’s floor dimensions.
The base should be the same size as the shed floor or very slightly smaller — never larger. An oversized base creates a shelf around the shed where rainwater running off the roof collects and sits against the timber, which rots the bottom rail.
| Shed size | Base area | Concrete at 100mm | Type 1 at 100mm |
|---|---|---|---|
| 6 × 4 ft | 1.8 × 1.2 m | 0.22 m³ | 0.45 t |
| 8 × 6 ft | 2.4 × 1.8 m | 0.43 m³ | 0.91 t |
| 10 × 8 ft | 3.0 × 2.4 m | 0.72 m³ | 1.51 t |
| 12 × 8 ft | 3.6 × 2.4 m | 0.86 m³ | 1.81 t |
| 12 × 10 ft | 3.6 × 3.0 m | 1.08 m³ | 2.27 t |
Run your own dimensions through the slab calculator for exact figures with wastage included.
Position and access
Before marking out, think about the finished shed. Leave at least 450mm clear around the sides and back for maintenance and for treating the timber. Check the door swing has room, and that the position doesn’t sit in a hollow where water collects.
Distance from boundaries matters too — most permitted development rules require outbuildings to be at least 2m from a boundary if they exceed 2.5m in height.
Marking out square
Set out with pegs and string. To get a right angle, use the 3-4-5 method: measure 3 units along one line and 4 along the other, and the diagonal between those points must be exactly 5 units.
Then check both diagonals of the rectangle. If they’re equal, the shape is square. If they differ by more than about 5mm, adjust before going further — a base that’s out of square gives you a shed that won’t sit properly and doors that don’t close.
What you need
| Item | For an 8 × 6 ft base |
|---|---|
| Concrete | 0.43 m³ (37 bags at 25 kg) |
| MOT Type 1 | 0.91 t (one bulk bag) |
| Formwork timber | 9m of 25 × 150mm |
| Pegs | 16 × 450mm |
| DPM | 6 m² |
| Plate compactor | 1 day hire |
| Mixer | 1 day hire |
Excavation
Dig to 200mm below finished base level for a 100mm slab on 100mm of sub-base, extending 150mm beyond the base on all sides so the sub-base supports the edges.
Remove all topsoil and roots. Organic material rots and settles, and anything left under the base becomes a soft spot later. Dig to firm subsoil even if that means going deeper.
Decide the finished height now. The base should sit slightly proud of the surrounding ground — 25 to 50mm — so water runs away from it rather than pooling against the shed.
Sub-base
Lay 100mm of MOT Type 1 in two 50mm layers, compacting each with a plate compactor. Four to six passes per layer, in overlapping runs, then again at right angles.
The finished surface should show no give underfoot at all. This layer is what stops the base settling, and it matters more than the concrete thickness.
Formwork
Use straight 25 × 150mm timber, pegged at 600mm centres with stakes driven outside the formwork. Set the top edge at exactly finished base level, because you’ll screed off it.
Check the formwork is level in both directions and check the diagonals again. Brace the outside well — wet concrete pushes outwards with real force and a bowed form leaves a base you can’t square up afterwards.
A shed base is normally laid level rather than with a fall. The shed keeps rain off it, and a level floor is easier to build on.
Membrane
A DPM isn’t required by regulation for an open shed, but it’s worth the few pounds. It keeps ground moisture out of the slab, which keeps the shed floor and everything stored on it drier.
Lay 300 micron polythene over the compacted sub-base, lapping any joints by 150mm and turning it up at the edges to be trimmed later.
Pouring and finishing
Place the concrete from the far corner and work back. Spread with a rake to slightly above the formwork, then tamp with a straight board across the forms to compact it and drive out air.
Screed level with a sawing motion, pulling the board back across both forms. Fill any low spots and screed again.
Once the surface water has gone, float it. A wood float finish gives a slightly textured surface with good grip, which suits a shed floor better than a smooth trowel finish.
Round the edges with an edging tool. For a base larger than about 3m in any direction, cut a control joint to divide it into roughly square panels.
Ordering the concrete
At under a cubic metre, a shed base sits awkwardly between the options. Bags are practical but tedious — 37 bags of 25 kg for an 8 × 6 base is a solid morning of mixing. A drum-mix delivery attracts a part-load charge that can double the cost. A volumetric truck is usually the sweet spot, since it bills only for what you use.
Mixing from ballast and cement is the cheapest route on materials: roughly one bulk bag of ballast and six bags of cement for the same base, at around half the cost of premix bags.
Curing and fixing the shed down
Cover with polythene and keep damp for at least seven days. Strike the formwork after 24 to 48 hours.
Wait three to four days before putting a light shed on, longer for anything substantial. Fix the shed down with expanding masonry anchors through the bottom rail into the slab — a shed that isn’t anchored will move in high wind.
Sitting the shed on treated timber bearers rather than directly on the concrete is worth considering. It lifts the floor clear of any surface water and lets air circulate underneath.
Common questions
How thick should a shed base be?
100mm of concrete on 100mm of compacted sub-base for a normal garden shed. Go to 125 or 150mm for a large workshop or if you’ll be storing anything heavy.
Should the base be bigger than the shed?
No — match the shed floor or make it very slightly smaller. An overhanging base collects roof runoff against the timber and rots the bottom rail.
Do I need planning permission for a shed base?
Not usually for a base under a normal garden shed within permitted development. Check the rules on outbuilding size, height and boundary distance before you start.
Can I lay a shed base on a slope?
Yes, by cutting into the high side and building the formwork up on the low side. On anything more than a gentle slope you may need a retaining edge, and the excavation gets substantially bigger.
How long before I can put the shed up?
Three to four days for a light timber shed. A week for anything heavier. The concrete keeps gaining strength for 28 days but is easily strong enough for a shed well before then.